1. Field of the Invention
The present invention relates to bipolar transistors and, more particularly, to a Schottky-clamped bipolar transistor with reduced self heating.
2. Description of the Related Art
A bipolar transistor is a well-known structure that has an emitter, a base connected to the emitter, and a collector connected to the base. The emitter has a first conductivity type, the base has a second conductivity type, and the collector has the first conductivity type. For example, an npn bipolar transistor has an n-type emitter, a p-type base, and an n-type collector.
An npn bipolar transistor turns on when the voltage on the p-type base exceeds the voltage on the n-type emitter by approximately 0.7V, thereby forward biasing the junction. When the base-to-emitter junction becomes forward biased, the operation of the bipolar transistor depends on the voltage present on the collector.
When the voltage on the n-type collector is greater than the voltage on the p-type base, the base-to-collector junction is reverse biased and the transistor enters an active mode of operation. In this mode, a large number of electrons flow into the p-type base from the n-type emitter across the forward biased junction. A large portion of these electrons are captured by the electric field that lies across the reverse-biased base-to-collector junction, and then collected by the collector.
Alternately, when the voltage on the n-type collector is less than the voltage on the p-type base by approximately 0.7V, the base-to-collector junction becomes forward biased and the transistor enters a saturation mode of operation. In the saturation mode of operation, the transistor provides high current conduction from the collector to the emitter.
The saturation mode of operation is not preferred for high-frequency applications because once the bias voltages change, it takes a relatively long period of time for the transistor to recover from operating in the saturation mode. Thus, due to the long recovery time, bipolar transistors which operate in the saturation mode typically have a limited operating frequency.
To prevent a bipolar transistor from entering into the saturation mode, a Schottky diode is commonly placed between the base and collector, such that the anode (input) of the Schottky diode is connected to the base and the cathode (output) of the Schottky diode is connected to the collector.
In operation, a Schottky diode typically has a forward voltage drop of approximately 0.3V-0.4V. Thus, when the base-to-collector junction of a bipolar transistor is clamped by a Schottky diode, the base can never rise more than 0.3V-0.4V above the collector. As a result, the base-to-collector junction of a Schottky-clamped bipolar transistor can never become forward biased and, as a result, can never enter into the saturation mode.
FIG. 1 shows a cross-sectional view that illustrates an example of a prior-art Schottky-clamped bipolar transistor 100. As shown in FIG. 1, Schottky-clamped bipolar transistor 100 includes a Schottky diode 110 and an npn bipolar transistor 112. As further shown in FIG. 1, Schottky-clamped bipolar transistor 100 utilizes a silicon-on-insulator (SOI) wafer which has been conventionally processed to have a bulk region 114, an n-single-crystal silicon layer 116, and a buried isolation layer 118 that lies between and electrically isolates single-crystal silicon layer 116 from bulk region 114.
In addition, the SOI wafer has also been conventionally processed to have a deep trench isolation (DTI) structure 120 and a number of shallow trench isolation (STI) structures 122. DTI structure 120 extends through single-crystal silicon layer 116 to touch isolation layer 118 and form a large number of fully-isolated single-crystal silicon regions 124, including a fully-isolated n-single-crystal silicon region 124-1 that supports Schottky diode 110 and a fully-isolated n-single-crystal silicon region 124-2 that supports bipolar transistor 112. The STI structures 122, in turn, include an STI ring 122-1 that is formed in fully-isolated single-crystal silicon region 124-1, and an STI region 122-2 that is formed in fully-isolated single-crystal silicon region 124-2.
As also shown in FIG. 1, Schottky diode 110 includes an n+ ring 130 and a p+ guard ring 132 that are formed in fully-isolated single-crystal silicon region 124-1 on opposite sides of STI ring 122-1. Schottky diode 110 also includes a metal ring 134 that touches the top surface of n+ ring 130, and a metal region 136 that touches the top surface of fully-isolated single-crystal silicon region 124-1 and p+ guard ring 132. Metal ring 134 and metal region 136 are commonly formed with a silicide, such as platinum silicide.
As additionally shown in FIG. 1, bipolar transistor 112 includes a collector structure 140. Collector structure 140, in turn, includes an n+ buried layer 142 that is formed in fully-isolated single-crystal silicon region 124-2 to touch the top surface of buried isolation layer 118, an n-well 144 that is formed in fully-isolated single-crystal silicon region 124-2 to extend down and touch n+ buried layer 142, and an n+ collector sinker region 146 that is formed in fully-isolated single-crystal silicon region 124-2 to extend down and touch n+ buried layer 142.
Bipolar transistor 112 also includes a p-type silicon germanium (SiGe) base 150 that touches the top surface of fully-isolated single-crystal silicon region 124-2, and an n-type silicon emitter 152 that touches the top surface of SiGe base 150. A SiGe base, which forms a heterojunction bipolar transistor (HBT), is commonly used in high-frequency applications.
As further shown in FIG. 1, Schottky-clamped bipolar transistor 100 includes a non-conductive layer 154 that touches the top surfaces of DTI structure 120, the STI structures 122, metal ring 134, metal region 136 SiGe base 150, and emitter 152. Transistor 100 further includes a number of contacts 156 that extend through non-conductive layer 154 to make electrical connections with metal ring 134, metal region 136, a silicided top surface of n+ collector sinker region 146, SiGe base 150, and silicon emitter 152. In addition, as schematically illustrated, metal region 136 of Schottky diode 110 is electrically connected to SiGe base 150, while metal ring 134 of Schottky diode 110 is electrically connected to the silicided top surface of n+ collector sinker region 146.
In operation, metal region 136 functions as the anode of Schottky diode 110 and silicon region 124-1 functions as the cathode of Schottky diode 110. In addition, n+ ring 130 functions as the cathode contact, while p+ guard ring 132 reduces the leakage current. As a result, when the voltage applied to SiGe base 150, and thereby metal region 136, rises above the voltage applied to n+ collector sinker region 146 by approximately 0.3V-0.4V, a current flows from SiGe base 150 to metal region 136 to n+ ring 130 to n+ collector sinker region 146. On the other hand, when the voltage applied to metal region 136 falls below the voltage applied to n+ collector sinker region 146, substantially no current flows from n+ ring 130 to metal region 136.
One of the drawbacks of Schottky-clamped bipolar transistor 100 is that DTI structure 120 significantly limits the lateral dissipation of heat, which limits the heat that can be generated by bipolar transistor 112 which, in turn, limits the operation of bipolar transistor 112. Heat is produced when current flows through a bipolar transistor. This type of heating, which is known as self heating, increases as the density of the current increases.
As bipolar transistors are scaled downward, the density of the current flowing through the transistors increases, which produces increasing levels of heat. Thus, as the transistors are scaled downward and the levels of heat rise through self heating, the inability to significantly dissipate heat laterally through DTI structure 120 affects the conductivity of the transistor and limits the safe operating area of the transistor.
One approach to reducing self heating is to increase the length L shown in FIG. 1, which is the distance from the edge of the shallow portion of the DTI structure 120 that lies below SiGe base 150 to the edge of the deep portion of the same DTI structure 120. FIG. 2 shows a prior-art graph 200 that illustrates a collector-emitter voltage V versus a collector current A for different values of the length L. As shown in FIG. 2, the upper portion of the collector current curve can be substantially improved (flattened out) by increasing the length L from 0.25 μm to 5.00 μm.
Although the collector current curve can be substantially improved by increasing the length L from 0.25 μm to 5.00 μm, increasing the length L increases the area or footprint of a fully-isolated single-crystal silicon region, such as fully-isolated single-crystal silicon region 124-2. However, increasing the footprint of fully-isolated single-crystal silicon region 124-2 is not a realistic solution when the goal is to reduce the footprints of the devices and the die, which includes reducing the footprint of the fully-isolated single-crystal silicon regions. Thus, there is a need for an approach to reducing the self heating experienced by a SiGe bipolar transistor in a fully-isolated single-crystal silicon region.